1. Field of Invention
The present invention relates to seismic sensor units, and is more particularly but not exclusively concerned with seismic sensor units for land applications. Such sensor units, also called geophones, are used for detecting seismic signals that propagate through the ground. The output signals of numerous such geophones are fed into a seismic survey telemetry system and transmitted to recording and data processing units. Processed seismic data are then presented in form of seismic traces in a seismogram that is useful as information about subsurface stratigraphy.
2. Description of Prior Art
Most conventional geophones consist of a case that houses and secures at least one internal electric seismic sensor element that senses seismic signals in the ground. Thus, in order to provide a sufficient acoustic and mechanically stable contact with the ground most geophones are equipped with a spike or the like that is attached to the geophone case and planted into the ground. Electric cable is fixed to the case as well, to allow each geophone to be connected to other geophones and to transmit output signals to data acquisition units. The most commonly used seismic sensor elements in conventional geophones are moving coil sensor elements with a large dynamic range and good resolution. But the frequency bandwidth of moving coil geophones is limited due to relatively low frequency resonance, ie in the region of 10 to 30 Hz, and on the high side due to spurious frequency noise. And the performance of such moving coil geophones strongly depends on the verticality of the planted geophone.
During seismic data acquisition, a group of several conventional geophones is typically deployed over a certain area and interconnected to each other via cable. Usually the desired seismic signals are contaminated by unwanted noise signals, such as horizontally travelling waves, so-called ground-roll, and random (incoherent) noise such as wind noise, rain noise, scratching of the geophone case by vegetation, geophone cable oscillation, etc. In order to attenuate such noise, the analog output signals of a group of closely spaced geophones are grouped together by adding them into a single analog seismic group signal before being further processed. In total, a very large number of conventional geophones is needed in a seismic survey to realise such an analog method of improving the signal-to-noise ratio of the desired signals. Today, typically 24 geophones are needed per seismic trace.
It is therefore an object of the present invention to overcome the above described drawbacks of conventional geophones and, in a preferred implementation of the invention, to provide a seismic sensor unit that allows the signal-to-noise ratio of the output signals of the seismic sensor units to be improved with, and despite of, a significantly reduced number seismic sensor units per seismic trace.
According to the present invention, there is provided a seismic sensor unit comprising: a case that houses at least one electric seismic sensor element and allows said sensor unit to be placed on and acoustically coupled to the ground; a cable, fixed to the case, to allow the supply of power to the sensor unit and to transmit the sensor unit output signal to other seismic sensor units and/or to a signal processing unit; wherein the case also houses a first electronic circuit in which the output signal of the sensor element is digitised, and a second electronic circuit which comprises a telemetry interface module that serves to exchange signals and data with a seismic data acquisition network, said second electronic circuit including a signal filter module and a logic and signal conditioning module which applies calibration coefficients to the sensor unit output signal.
The major advantage of a seismic sensor unit according to the invention is that it allows digitised seismic data to be obtained from each individual sensor unit, which data can be digitally filtered in an adaptive manner, so that less sensor units are required per seismic trace. Such processing can be performed remotely, either in electronic boxes along the seismic line, in a central system computer or in a data processing centre. Because of the digital output signals from the seismic sensor units, long analog signal cables, which in conventional seismic systems cause signal disturbances due to loss and noise picked up along the cable, are no longer needed. All this reduces the cost of seismic data acquisition, inter alia by improving acquisition logistics, and improves seismic data quality.
Further embodiments of the seismic sensor unit according to the invention make use of feed-back controlled accelerometers as seismic sensor elements. In particular, the use of digital accelerometers that are capable of measuring DC signals allows the determination of the gravity component parallel to the sensor axis. This gravity measurement can be performed during a test period either before or after the seismic data acquisition. Quality control of acquired seismic data can be provided even during the measurement phase in the field.
Another embodiment of the seismic sensor unit according to the invention applies recently developed manufacturing technology of electronic components and sensor elements. The major advantage that can be thereby achieved is that the seismic sensor unit will be of a smaller size than geophones available today, and of less weight. Even in view of geophysical aspects, such smaller size of the seismic sensor element is advantageous, because it renders the seismic sensor less sensitive to non-linear noise.